This invention is concerned with the reduction of undesirable substances in the exhaust gases of internal combustion engines. It is well known that the types and amounts of substances present in engine exhaust is greatly affected by the ratio of air to fuel in the mixture supplied to the engine. Rich mixtures, with excess fuel, tend to produce higher amounts of hydrocarbons and carbon monoxide; whereas lean mixtures, with excess air, tend to produce greater amounts of oxides of nitrogen. It is well known that exhaust gases can be catalytically treated to reduce the amounts of these undesirable components, the catalytic treatment including oxidation of carbon monoxide and hydrocarbons and reduction of nitrogen oxides.
It has been suggested that both the oxidation and reduction necessary for the minimization of these undesirable exhaust constituents can be achieved with a single catalytic device, provided that the air-fuel mixture supplied to the catalytic converter is maintained within a narrow range at stoichiometry, the ratio containing fuel and oxygen in such proportions that, in perfect combustion, both would be completely consumed. If air-fuel ratio is defined as the amount by weight of air divided by the amount by weight of fuel, there is a narrow range of about 0.05 air-fuel ratio units about stoichiometry in which conversion efficiency is very high for both oxidation and reduction. However, this range, termed the "converter window", is too narrow to be maintained by any conventional open loop fuel control system; and conversion efficiency drops dramatically for the different undesirable exhaust constituents on either side of the window.
Therefore, it has been suggested that a closed loop fuel control system, in which the air-fuel ratio of the mixture supplied to the engine is controlled by a feedback signal from a zirconia sensor exposed to exhaust gases, can maintain the gases supplied to the converter within the converter window. However, the design of such a control system must meet a number of requirements. The system must be stable to maintain continual control and not go into oscillation. On the other hand, the system must be quick reacting and characterized by small overshoot, so that the minimum time is spent outside the converter window. Such a system would desirably be applicable to engines using carburetors as well as those using fuel injection.
A number of closed loop fuel control systems have been proposed, but none are completely satisfactory. Most use a zirconia sensor exposed to engine exhaust upstream from the converter and use proportional or integral control in the feedback loop. Such systems do maintain some control over the engine operating point but tend to drift out of the converter window over time as a result of changing sensor characteristics and other factors.